Yawning, breathing and stretching in humans and other vertebrates

Brainerd, E.L. Hoogendyk, T.A. Moomaw, H.A. and Townsend, P.J.

We have collected video of yawn-like behaviors in cartilaginous and ray-finned fishes, a lungfish, salamanders, caecilians, mammals, turtles, lizards, an alligator and birds. All share a similar jaw movement pattern during yawning: a slow opening phase, followed by a slower opening phase or plateau, followed by rapid closure. This similarity leads us to conclude that all of these behaviors may reasonably be called yawning, and the conservation of this behavior over 400 MY of vertebrate evolution suggests that it may serve some physiological function. Yawning in humans is associated with one or more large breaths. A common belief holds that the function of yawning in humans is to provide more oxygen to the brain. This probably is not the function of yawning in fishes and amphibians, as yawning appears to interrupt rather than augment their breathing. We measured tidal volume and minute volume during normal breathing and yawning in five human subjects. We found that tidal volume is larger during yawns (ANOVA, P<0.001), but that minute volume is not larger (P>0.05) because of apnea that is associated with the yawn. We also found that exposing humans to low oxygen (13%) did not increase the frequency of yawning (paired t-test, P>0.05). Based on these results, we reject the hypothesis that the function of yawning is to increase oxygen delivery. Yawning also has been thought to serve a similar physiological function to stretching. We measured the stretching movements of fins and limbs in association with yawns, and found that they follow the same movement pattern as jaw opening and closing. This lends support to the existing hypothesis that yawning and stretching serve a similar function, but the detailed nature of this function remains unclear. (NSF IBN-9875245)